The present invention relates to the biomedical arts, particularly implantable nerve cuffs for both stimulating and monitoring nerve activity. The present invention finds application in electrodes embedded in nerve trunks or other small tissue strands and will be described with reference thereto. It is to be appreciated, however, that the invention is also applicable to medicinal infusers and other implanted biomedical devices for introducing, monitoring, or removing matter, fluids, or energy.
Functional electrical stimulation of the nervous system has been shown in recent years to offer great hope in restoring some degree of lost sensory and motor function in stroke victims and individuals with spinal cord lesions. In certain specialized applications, such as in the treatment of sleep apnea syndrome, it is necessary to simultaneously monitor as well as generate electrical signals in nerves. A generic sensor/effector electrode is suggested in U.S. Pat. No. 4,830,008 to Meer. That patent, however, provides no specific details regarding the electrode.
Methods and apparatus in which functional electrical stimulation and/or recording are utilized to restore a particular function broadly include:
(1) the use of surface electrodes to activate nerves in a general region of interest; PA1 (2) the use of intramuscular electrodes, also to activate nerves in a general region; PA1 (3) the use of nerve cuff electrodes placed around specific nerves of interest and used to activate individual nerves specifically; and, PA1 (4) the use of regeneration-type neural interfaces including microelectrode arrays.
The third alternative, i.e. cuff electrodes, offers advantages over the first two in that it requires the least amount of stimulating current to produce a desired effect and hence injects the least amount of charge into the tissue itself. Because the use of nerve cuff electrodes requires delicate surgery and may damage the nerves, they are usually contemplated only when excitation of specific, isolated muscles is desired, or when unidirectional propagation action potentials are required.
The prior art cuff electrodes can be generally classified as split-cylinder type or self-curling coil type. The split-cylinder type cuff electrode typically includes a cylinder of dielectric material defining a bore therethrough having sufficient diameter to receive a nerve trunk to be electrically stimulated. Two or three annular electrodes are positioned on the inner surface of the bore for applying an electrical stimuli. The electrical stimuli, for example, may be used to provide functional electrical stimulation, to block neural nerve impulses traveling along the nerve trunk, or to cause other effects.
Examples of cylindrical type cuff electrodes and their use include U.S. Pat. Nos. 4,608,985, 4,628,942 and 4,649,936, all assigned to the assignee of the instant application.
The self-curling type prior art cuff electrodes typically include a self-curling sheet of non-conductive material biased to curl into a tight spiral. A pair of conductive strips are disposed on the self-curling sheet extending peripherally around the diameter of cuff passage. The conductive segments may be electrically conductive for applying electrical impulses or fluid conductive for infusing or extracting medications. An example of this type cuff electrode is U.S. Pat. No. 4,602,624, assigned to the assignee of the instant application.
In use, a first edge of the self-curling sheet is disposed adjacent a nerve trunk which is to receive the cuff therearound. The self-curling sheet is permitted to curl around the nerve forming an annular cuff. The effectiveness of this type of cuff is limited, however, due to the placement of the conductive material on the nerve surface, rather than placement within the interior portions of the nerve. Also, although presenting an improvement over the split cylinder cuff electrode type described above, the self-curling type electrodes tend to interfere with normal swelling and movement of the nerve. Damage to the nerve fibers can result when the self-curling cuff lodges too tight around the nerve preventing nutrients, blood and other critical fluid flow.
Another prior art approach to electrical stimulation of the nervous system is taught by Nannini, et al. in Muscle Recruitment with Intrafascicular Electrodes, I.E.E.E. Transactions on Biomedical Engineering, Vol. 38, No. 8, August 1991. There, a bipolar, intrafascicular electrode is used to penetrate the perineurium membrane and is advanced into an individual fascicle of a nerve. A bipolar electrode pair is formed on two small insulated wires. A Tungsten needle is then used to thread a first (inside) wire of the electrode through the nerve fascicle for about 1 cm. with an exposed tip portion of the wire entering this region. The second (outside) wire is not threaded through the nerve but is placed on the outside of the fascicle. The distal ends of the two wires are fastened to the fascicular endoneurium with a suture. The proximal end is secured in place by suturing a loop emerging from a piece of silastic tubing to the epineurium. The tubing is led to the skin and the wound is closed, leaving the two wires accessible. Although this method is highly invasive and can permanently damage the nerve through penetration of the perineurium, it demonstrates the advantages, e.g. effectiveness, of intra-fascicular recording and stimulating electrodes.
Regeneration-type neural interfaces have been used as another form of recording and stimulating electrical activity from within a nerve. One example is U.S. Pat. No. 4,623,355 to Sawruk. The basic idea behind this type of interface is to manufacture a thin diaphragm with many small holes that can be positioned between the cut ends of a peripheral nerve. The nerve is left to regenerate over time through the many small holes in the diaphragm. The holes are formed by mechanical or laser drilling, or by semiconductor fabrication techniques including wet and dry etching of silicon substrates. Sophisticated interfaces include active electronics on the devices. However, although these devices are theoretically attractive, actual functionality falls short primarily because the nerves tend to regenerate around the interface rather than through it.
Our earlier co-pending application Ser. No. 08/138,237, filed Oct. 15, 1993 teaches a slowly penetrating interfascicular nerve cuff electrode which includes a split cylinder cuff embodiment arranged with "frayed" ends formed from non-conductive surgical tube material. The ends comprise a plurality of spring members formed by longitudinal slices into the ends of the cylinder. Each of the plurality of spring members carries at least one fin member construction which in turn supports at least one medication or electrical energy conductive member. Each of the spring members are self-biased to slowly drivingly urge the fin members into the nerve trunk or other body tissue at a predetermined rate. The conductive members carried on the fin members are thus correspondingly driven into the nerve tissue. The predetermined driving rate as well as the shape of the fins are particularly selected to be slow enough and blunt enough, respectively, so that the axons within the nerve trunk are displaced rather than destroyed or pierced by the at least one medication or electrical energy conductive member. Although the inner diameter of the cuff embodiments of our earlier patent are necessarily slightly greater than the outer diameter of the target tissue, it would be desirable to provide additional circumferential elasticity for anticipated excessive swelling beyond the limits of the cuff inner diameter boundary. A direct advantage to providing additional circumferential elasticity in the cuff, as an alternative to providing a gap or space to accommodate the swelling, is an increased effectiveness of the cuff.